1. Field of the Invention
The present invention relates generally to CDMA communication systems, and in particular, to a device and method for controlling slotted mode communication.
2. Description of the Related Art
In a code division multiple access (CDMA) communication system, a forward link includes a pilot channels, a sync channel, a paging channel and a traffic channel. By using these channels, a base station and a plurality of mobile stations acquire sync of PN spreading codes and establish call channels.
Upon power on, the mobile station acquires a pilot signal, having the highest power, out of plurality of pilot signals received over the forward channels, and demodulates a sync channel message to acquire a reference time. Once a reference time is established the mobile station will thereafter perform demodulation. After receipt of the sync channel message, the mobile station can operate in a slotted mode wherein the mobile station periodically monitors a paging channel to check whether a paging message has been transmitted from a base station. Upon receipt of a paging message from a base station, the mobile station makes a transition to an active state where the mobile station can provide the call service using a traffic channel. However, if the mobile station does not receive the paging message, the mobile station enters a sleep mode where it cuts off a power supply voltage being provided to demodulation-related hardware, such as an RF (Radio Frequency)/analog part, searcher, finger and combiner, to disable the demodulation-related hardware.
Upon entering the sleep mode, the mobile station checks whether a paging message for it has been received from a base station, as stated above. The base station can transmit a paging message to the mobile station in either a slotted mode or a non-slotted mode. In either mode the message is transmitted in a unit of superframe having a length of 80 ms, which is called a slot. The 80 ms superframe includes information about a paged mobile station, information about the base station and a list of neighbor sectors of the mobile station.
To determine whether a paging message for a specified mobile station has been received or not, the mobile station should monitor paging channel slots transmitted from the base station. If the mobile station does not know when the paging message for it will be received, the mobile station should monitor all the slots that the base station transmits. Such a paging mode is called a non-slotted mode in which a time slot, at which a paging message for a specific mobile station will be transmitted, is not specified, requiring that the mobile station s monitor all the slots.
However, when a paging message for a specific mobile station is transmitted only at a predetermined specific slot, the mobile station can monitor only the corresponding specific slot and, instead of demodulating a paging channel message for other time slots, and cut off a power supply voltage being provided to the CDMA demodulation-related hardware to enter the sleep mode. As stated above, in the slotted mode, the mobile station can operate in the sleep mode for the period of time that the receiving stage is disabled. FIG. 1 illustrates a message slot assigned to a specific mobile station transmitted in one cycle from a base station for a communication system operating in the slotted mode, and a monitoring time for which the mobile station monitors the slot assigned to itself.
Referring to FIG. 1, a base station transmits a paging channel message for a specified mobile station at a specific slot for the paging channel (11). The specified mobile station then provides a power supply voltage to an RF/analog circuit beginning at a time preceding the start time of the specific slot for the paging channel (12), and enables a searcher (13). After search of the base station using the searcher, the mobile station assigns a finger to receive the paging channel message (14).
FIG. 2 illustrates a method for receiving the above-stated paging channel message. FIG. 2 shows a case where a sixth slot is assigned to the mobile station and a slot cycle is 16 slots (i.e., 1.28 seconds). Further, for a duration A, the mobile station is awaken from the sleep mode several tens of milliseconds before the sixth paging slot, for demodulation of the paging slot, provides power to the RF/analog circuit, searches base station pilot signals, and assigns multipath signals for an acquired pilot signal to a finger.
In an existing IS-95 system, a base station can operate either in the slotted mode or the non-slotted mode, but it is a system designer""s option whether to support the slotted mode or the non-slotted mode. However, when the base station operates in the slotted mode, the mobile station can increase the standby time by demodulating the paging channel at the pre-assigned time slot, taking into consideration the limited batter capacity of the mobile station. That is, the mobile station monitors only a slot at which a paging message for it is to be received and, for the other time slots, cuts off the power supply voltage of the RF/analog circuit and the demodulation-related module thereof, thereby to increase the standby time.
As previously stated, FIGS. 1 and 2 illustrate a paging slot transmission time of a base station and an operating duration of a mobile station in an existing IS-95 system, wherein the base station and the mobile station both operate in the slotted mode. For demodulation of a paging slot assigned to it, the mobile station is awaken from the sleep mode several tens of milliseconds before the pre-specified paging slot, to search a base station from which the paging slot is to be transmitted.
The RF/analog circuit of the mobile station is awaken from the sleep mode to operate in a stable state (12). Thereafter, the searcher of the mobile station is awaken from the sleep mode to detect correlation and energy between a received signal and a PN (Pseudo Noise) spreading code of the mobile station so as to detect a pilot signal received with sufficiently high energy (13). The mobile station repeats the steps of energy detection and comparison of the detected energy with a threshold for those sectors registered in an active sector list and a neighbor sector list until a single pilot signal is acquired which exceeds the threshold in energy. Upon failure to acquire a pilot signal having sufficiently high energy, the mobile station cannot normally demodulate the paging slot. However, upon receipt of a pilot signal having a sufficiently high energy, the mobile station assigns the multipath components of the acquired pilot signal to a finger to attempt demodulation of the paging slot (14). Subsequent to a paging message being detected through demodulation, the mobile station then makes a transition to a traffic channel state to establish a communication link. However, when a paging message is not detected, the mobile station enters the sleep mode again till the next paging slot to save power.
Here, the mobile station determines the period between the slots at which a paging message for a specific mobile station is to be transmitted and notifies the determined period to the base station through a registration procedure. The paging slots are assigned with unique numbers from 0 to 2047, and a slot to be first monitored is determined by a Hash function unique to each mobile station.
A mobile station may alternately determine a period between slots by changing a slot cycle index (SCI) according to a reacquisition algorithm for a pilot signal. The relationship between an actual inter-slot period and the SCI is defined as
Slot Period=16*2SCI [slots], where SCI=0,1,2,3,4,5,6 and 7.
As illustrated in FIG. 2, when the sixth slot is to be first monitored and SCI=0 (i.e., the period between slots is 16), the mobile station enters the sleep mode for the slot time from the sixth slot to the twenty-second slot to save the power.
In enabling the mobile station to operate in the slotted mode, it is important for the mobile station to perform a reacquisition procedure for searching a pilot signal for those sectors registered in the active sector and neighbor sector lists to monitor the next paging slot, after completion of the sleep mode. That is, it is necessary to acquire timing upon completion of the sleep mode to acquire an active sector for receiving a paging message. In implementing a receiver of the existing IS-95 system, 3 PN ROLL (1 PN ROLL (the period of PN spreading code)=26.7 ms), i.e., 80 ms is assigned for the above operation. The mobile station should complete determination of an active sector for data demodulation and accurate timing acquisition, within 80 ms. However, when the mobile station fails to determine an active sector and cannot perform reacquisition due to a bad channel condition, the 80 ms superframe is not enough to perform the reacquisition procedure for 20 neighbor sectors in maximum. In this case, the mobile station cannot correctly receive the paging message. That is, in the existing IS-95 system, reduction in reacquisition time to achieve a battery saving may cause loss of sync and failure to receive a paging message. In particular, as the neighbor sectors to be managed increase in number, it is difficult to acquire a pilot signal from a neighbor sector, having a good receiving condition, within 80 ms.
Further, the IS-95 system has another problem of idle search for set maintenance, performed for the 80 ms slot time where a paging channel message is received. In the IS-95 system, the mobile station performs the search operation for one sector in the active sector or neighbor sector list in the following pattern, while the mobile station stays in the paging state.
Axe2x86x92N1xe2x86x92Axe2x86x92N2xe2x86x92xe2x80xa2 xe2x80xa2 xe2x80xa2 xe2x86x92Axe2x86x92R
Here, R represents remaining sectors other than the active sectors and the neighbor sectors.
The active sector and neighbor sector lists are updated by performing the pilot channel search operation in the above stated order. In order to deal with a variation in channel condition of a mobile station and a change of adjacent base stations, the search operation should be frequently performed. However, an increase in the SCI increases the time of the sleep mode. For this reason, it is difficult to frequently perform the idle search during the 80 ms paging slot. Therefore, it is not possible to search and manage the active sectors or neighbor sectors by rapidly responding to the variation in peripheral environments, which causes a difficulty in acquiring a good active sector or neighbor sector in a reacquisition procedure for receiving a paging slot. As a result, it is not possible to demodulate the paging message. That is, it might be difficult to perform an effective set maintenance for an SCI having a sufficiently high value. However, maintaining the SCI at a low value does not coincide with the goal of power saving in the slotted mode. That is, reducing the time, for which the sleep mode continues, is disadvantageous to power saving. Further, in a micro-cellular or pico-cellular system having small-sized cells, the problem becomes more serious. That is, when the mobile station is awaken from the sleep mode to monitor the paging slot, since the mobile station has already passed many cells, the previous list of the neighbor cells may become useless. As a result, the mobile station may lose sync and therefore cannot respond to the paging, so that it should return to an initial sync acquisition state.
As described above, operation in the paging channel state and the slotted state, and a transition method and a protocol between the two states suffer from multiple problems making it an inappropriate solution for an IMT-2000 system environment.
It is, therefore, an object of the present invention to provide a device and method for performing communication in a slotted mode in a CDMA communication system.
It is another object of the present invention to provide a device and method for performing set maintenance and reacquisition by employing a new pilot signal transmission method and a new slotted mode to secure an effective slotted mode operation in a CDMA communication system.
It is a further object of the present invention to provide a device and method for enabling a mobile station to acquire and manage signals from an increased number of neighbor base stations in a CDMA communication system employing a slotted mode.
It is still another object of the present invention to provide a device and method for acquiring signals from neighbor base stations by a mobile station with reduced power consumption and complexity in a CDMA communication system employing a slotted mode.
It is a further object of the present invention to provide a device and method for acquiring signals from neighbor base stations by a mobile station with reduced power consumption and complexity which is in an idle state, in a CDMA communication system.
It is still another object of the present invention to provide a device and method for acquiring and managing signals from an increased number of neighbor base stations to save battery power by enabling a mobile station to store received signals in a memory thereof in a CDMA communication system.
It is still another object of the present invention to provide a device and method for storing sample data of a pilot signal in a memory during a sleep mode to perform a search operation and performing sector selecting and finger assignment for demodulation of a quick paging channel according to the search result, by employing a new pilot signal transmission method in a CDMA communication system adopting a slotted mode.
It is still another object of the present invention to provide a device and method for increasing energy of a pilot signal for a short time to perform set maintenance by enabling only a searcher during a sleep mode for that time, and for saving power by increasing a probability of pilot signal acquisition and thus reducing an acquisition time in a CDMA communication system employing a slotted mode.
To achieve the above object, a device for communicating a common channel message in a CDMA communication system includes a base station transmitter including a specific channel transmitter for outputting a channel signal with second transmission power at a predetermined time duration and outputting the channel signal with first transmission power at other time duration, and a common channel transmitter for transmitting a common channel message at an assigned slot duration; and a receiver of a mobile station, wherein a power supply voltage is provided to the receiver at the duration where the specific channel is received with the second transmission power and the assigned slot duration to enable the receiver, wherein the power supply voltage to the receiver is cut off at other duration so as to allow the receiver to operate in a sleep mode.